KR101125909B1 - Ship-building steel with excellent general corrosion and pitting corrosion resistance at low ph chloride solution - Google Patents

Abstract

The present invention relates to a marine steel material having excellent front and local corrosion resistance in a strong acid salt solution, more specifically in a solution environment in which the basicity of the brine is lowered due to the brine and corrosion contained in the crude oil, such as the bottom of the oil tank of the tanker It relates to marine steels having excellent front and local corrosion resistance.

The steel for ships of this invention is C: 0.02-0.2 weight%, Si: 0.05-1.5 weight%, Mn: 0.2-2.0 weight%, P: 0.03 weight% or less, S: 0.03 weight% or less, Cu: 0.05-1.0 weight %, Al: 0.1 wt% or less, N: 0.001-0.01 wt%, Ni: 0.05-3.0 wt%, Cr: 0.02-1.0 wt%, Mo: 0.02-0.5 wt%, W: 0.02-0.5 wt% and Ca : At least one element selected from the group consisting of 0.0005 to 0.01 wt%, balance Fe and other unavoidable impurities, wherein C, Si, Mn, Cu, Ni, Cr, Mo, and W are 1.5> C + 2.5 Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr has a composition that satisfies the relationship.

Local corrosion, formula, ship's steel, strong acid, brine solution

Description

SHIP-BUILDING STEEL WITH EXCELLENT GENERAL CORROSION AND PITTING CORROSION RESISTANCE AT LOW PH CHLORIDE SOLUTION}

The present invention relates to a marine steel material having excellent front and local corrosion resistance in a strong acid salt solution, more specifically in a solution environment in which the basicity of the brine is lowered due to the brine and corrosion contained in the crude oil, such as the bottom of the oil tank of the tanker It relates to marine steels having excellent front and local corrosion resistance.

Among various steels used in ships, especially steels used in oil tankers for oil tankers, very severe corrosion damage occurs due to the environment inside the oil tanks. Inside the crude oil tank, various types of corrosion progress due to volatile and mixed seawater in crude oil, inert gas sent into the tank for explosion prevention of salt in oilfield salt, and condensation due to internal temperature difference. It is much larger than that. In particular, induction bottom plate of the crude oil tanker has a large number of holes (Pit) of less than 50mm in diameter, the growth rate of the cultivation is up to 4mm / y. Unlike the bottom plate of a crude oil tank, the top plate has a corrosion rate of up to 0.3 mm / y, which does not significantly exceed the average wear rate of 0.1 mm / y due to corrosion considered in hull design.

In order to avoid such corrosion damage, some anticorrosive coating of crude oil tank material has been implemented, but the initial coating cost and future repainting cost are incurred, and in some paint defects, local corrosion may be further promoted. In addition, in consideration of the thickness loss due to corrosion, if the steel sheet thickness is thicker, not only the steel cost but also the weight of the ship itself becomes heavy, causing various problems such as the increase of fuel consumption. There is a demand for development of excellent marine steels.

Differences in the type of corrosion and the rate of corrosion by parts in the crude oil tanks result from the differences in the corrosion environment and the mechanism of corrosion. In the crude oil tank top, corrosion is caused by the reaction of H ₂ S gas evaporated from crude oil and CO ₂ , SO ₂ , O _2, etc. among inert gases injected for explosion protection with condensation formed on the steel surface due to temperature difference. On the other hand, the bottom plate is corroded by salt water contained in the crude oil or seawater introduced from the surrounding area, but the corroded portion is limited to the portion where the oil coating layer is defective. As it progresses, the brine in the corrosive acid is acidified. As the hydrogen ions generated by corrosion deteriorate with chlorine ions in the brine and continue to stagnate at the corrosion sites, the local corrosion sites become more acidic and the corrosion speed becomes faster with acidification, so that they grow in the form of vegetation with a very fast corrosion rate. do. In fact, the analysis of the solution collected from the inside of the plant found in the ship in operation showed a very low acidity with a pH close to 1.0. Therefore, the steel used as a crude oil tank preferably exhibits excellent corrosion resistance in a strong acid brine atmosphere.

In order to improve corrosion resistance of ship steels, the following techniques have been proposed.

Japanese Patent Application Laid-Open No. 2000-17381 proposes a shipbuilding steel having excellent corrosion resistance in an environment of use such as ship shell, ballast tank, cargo oil tank, photocargo hold, etc. %: C: 0.01 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.05 to 2.0%, P: 0.10% or less, S: 0.001 to 0.10%, Cu: 0.01 to 2.00%, Al: 0.005 to 0.10% , Mg: 0.0002 to 0.0150%, and the remainder is Fe and inevitable impurities, shipbuilding corrosion resistant steel, preferably Ni, Cr, Mo, W, Ca, REM, Ti, Nb, V , B, Sb, Sn is a steel containing an appropriate amount of one or more.

However, the oil for transporting crude oil having excellent corrosion resistance described in Japanese Patent Application Laid-Open No. 2001-17381 is effective in reducing the speed of local corrosion occurring at the bottom plate because the content of Si in its composition is limited to 0.4% or less. In addition, since Ni, Cu and Cr are limited to more than 0.5%, it may cause problems such as slab surface cracking in the manufacture of steel, and the amount of alloy added to ensure corrosion resistance may be excellent in corrosion resistance. There is a problem that is difficult. In addition, there is a disadvantage that the weldability is poor because the amount of alloying elements is increased.

Further, Japanese Patent Laid-Open No. 2001-214236 proposes a steel having excellent corrosion resistance when storing liquid fuel and raw fuel such as crude oil and heavy oil in crude oil tankers and oil tanks. : 0.003 to 0.30%, Si: 2.0% or less, Mn: 2.0% or less, Al: 0.10% or less, P: 0.050% or less, S: 0.050% or less, In addition to this, Cu: 0.01 to 2.0%, Ni: 0.01 7.0%, Cr: 0.01-10.0%, Mo: 0.01-4.0%, Sb: 0.01-0.3%, Sn: 0.01-0.3%, any one or two or more kinds are added, and the balance is Fe and unavoidable impurities. Corrosion resistant steel for crude oil and heavy oil storage is described. However, Sb and Sn are mainly added to improve the tensile strength and have a slight effect on corrosion resistance while reducing elongation and impact value. In addition, since both elements are low melting points and cause hot brittleness, tempering brittleness, low temperature brittleness, the addition of these elements may cause problems in the steel manufacturing process.

Another proposal for corrosion resistant steel is Japanese Patent Application Laid-Open No. 2002-173736, which provides steel and a method of producing corrosion resistance even under the environment of a tank transporting and storing crude oil. Steels provided by C: 0.001 to 0.20%, Si: 0.10 to 0.40%, Mn: 0.50 to 2.0%, P: 0.020% or less, S: 0.010% or less, Al: 0.01 to 0.10%, Cu: 0.5 to 1.5 %, Ni: 0.5 to 3.0%, Cr: 0.5 to 2.0%, or 1.0 ≦ 0.3 Ni + 2.0 Cr-0.5 Cu ≦ 3.8 (where Ni, Cr, Cu: content of each element (mass% Hot rolling is performed on a steel material having a satisfactory composition, followed by cooling at a cooling rate of 0.1 to 20 ° C./sec. In addition to the above composition, the steel has a composition which may further include one or two or more selected from Mo, Ti, Nb, V, and B, and one or two selected from Zr and Ca.

However, the present invention, like Japanese Patent Publication No. 2001-17381, has a negative effect on the reduction of the propagation speed of local corrosion caused by the bottom plate because it limits Si to 0.4% or less, which is advantageous for improving the corrosion resistance in the crude oil tanking environment. Since Cr is limited to 0.5% or more, it may cause problems such as surface cracks of slabs in the manufacture of steel, and there is a problem in that corrosion resistance cannot be obtained for cost due to the amount of alloy added.

In addition, Japanese Patent Application Laid-Open No. 2003-82435 proposes a steel material for low-cost cargo oil tanks with excellent corrosion resistance. C: 0.01 to 0.3% Si: 0.02 to 1% Mn: 0.05 to 2% P: 0.05% or less : 0.01% or less Ni: 0.03 to 3%, consisting of balance Fe and impurities, and additionally Mo, Cu, Cr, W, Ca, Ti, Nb, V, B, Sb, Sn and Al as needed. We propose steel having composition to include. In another embodiment, C: 0.01 to 0.3% Si: 0.02 to 1% Mn: 0.05 to 2% P: 0.05% or less S: 0.01% or less Ni: 0.01 to 3% Cu: 0.01 to 2% Cr: 0.05% Less than 30 inclusions per cm ² containing Al: 0.07% or less, remainder Fe and impurities, and having a particle size exceeding 30 μm, and C, which is the mass percentage of Ap and carbon, which is the percentage unit of pearlite in the structure. The steel is proposed to satisfy Ap / C≤130.

However, the above publication said that the addition of Cr is not preferable because it is harmful to the corrosion resistance in the crude oil tank environment, and not added to 0.05% or less, which is not applicable under strong acid brine conditions, and sufficient corrosion resistance by not adding Cr There is a problem that strength improvement cannot be obtained.

In addition, Korean Patent Laid-Open Publication No. 2005-0008832 shows excellent front and local corrosion resistance against crude oil corrosion generated in steel tanks, and also produces a corrosion product (sludge) containing solid S. To provide crude oil tank steel for welded structure, crude oil tank steel production method, crude oil tank oil and crude oil tank method, which can suppress the pressure of the welded structure. %: C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1-2%, P: 0.03% or less, S: 0.007% or less, Cu: 0.01-1.5%, Al: 0.001-0.3%, N : Cr: 0.001 to 0.01%, Mo: 0.01% to 0.2%, W: 0.01% to 0.5% or more, and more preferably Crude Oil by satisfying the solid solution Mo + solid solution W = 0.005% Corrosion products (sludge) which exhibit full corrosion resistance and local corrosion resistance in oil bath environment and also contain solid S The steel to suppress the generation can be seen is provided.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

However, the Japanese Unexamined Patent Publication No. 2003-82435 also suggests that the addition of Cr is limited to 0.05% or less because it is harmful to corrosion resistance, but the addition of Cr is a crude oil tank environment. Since it is not harmful to the corrosion resistance at all, it is advantageous to improve the corrosion resistance and the strength, there is a problem that can not be obtained through the addition of Cr to limit the corrosion resistance.

In addition, Japanese Patent Application Laid-Open No. 2005-171332 contributes to the extension of the repair repainting life of ship ballast tanks and to the reduction of repair repainting work, and has excellent corrosion resistance to avoid deterioration of weldability, weld part toughness, and rising manufacturing cost. According to the above publication which provides steel for ship ballast tanks, zinc-rich primer-coated steel is coated with zinc-rich primer on the surface of the descaled steel, wherein the steel is, in weight%, C: 0.03-0.2%, Si: 0.5% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.005 to 0.06%, Ni: 0.1 to 1.0%, N: 0.0020 to 0.0065%, Ti: 0.005 to 0.024 It is described that it can have corrosion resistance by having the composition which consists of%, and which consists of remaining thing Fe and an unavoidable impurity.

However, the steel provided in the present invention is not only used in the sea water atmosphere, but also to apply a zinc rich primer on the surface of the steel, there is a distance from the steel used in the strong acid salt water atmosphere without coating as in the present invention. In addition, the steel of the present invention also has a low Si content and does not add Cr, so there is a problem that sufficient corrosion resistance cannot be obtained in a strong acid brine atmosphere.

Japanese Patent Laid-Open Publication No. 2005-290479 proposes a steel material for a crude oil tank bottom plate having excellent local corrosion resistance even when used without application of an anti-rust paint, and as a chemical component, in mass%, C: 0.001∼ 0.20%, Si: 0.01-1.0%, Mn: 0.1-1.5%, P: 0.03% or less, S: 0.01% or less, Cu: 0.1-1%, Ni: 0.01-2%, Cr: Crude oil comprising 0.1 to 4% of Mo: 0.001 to 1% and having a composition consisting of residual Fe and unavoidable impurities, and the value of Pcm represented by the following formula (1) is 0.22 or less. Excellent for use in the bottom plates of transport tanks or storage tanks of crude oil is steel for crude oil tank bottom plates with local corrosion resistance. However, the steel materials described in the above publications contain Cu, which is an element having the greatest influence on the corrosion resistance, and thus there is a problem in that sufficient corrosion resistance cannot be secured when Cu is not added.

Korean Patent Publication No. 2006-0069937 is an example of steel having sufficient corrosion resistance even without coating. This publication is a marine corrosion resistant steel material that can be put into practical use even without coating or electric corrosion protection, especially in a wet atmosphere such as an upper portion of a ballast tank or a crude oil tank upper deck where electric corrosion protection does not work. The marine steels provided in the above publications, which provide excellent marine durability, and the like, provide C 0.01 to 0.20% (hereinafter, “%” means mass%), Si 0.01 to 0.50%, and Mn 0.01 to 2.0. %, Al 0.05 to 0.50%, Cu 0.01 to 5.0%, Cr 0.01 to 5.0%, respectively, except P 0.020% or less (including 0%) and S 0.010% (including 0%), respectively, remaining amount It is characterized by consisting of Fe and unavoidable impurities.

However, the steel disclosed in the above-mentioned Korean Patent Publication No. 2006-0069937 is a steel material that can be put into practical use even without coating or electrical corrosion prevention, and is used in the upper deck of the ballast or crude oil tank, so that the object of the present invention is As described above, the steel is not used while being immersed in a solution, but is used in a wet atmospheric atmosphere, and thus is different from the steel of the present invention. In addition, since Si is limited to 0.5% or less and Mo and W, which are helpful in improving strength and corrosion resistance, are not added, there is a problem in that sufficient corrosion resistance cannot be obtained.

The present invention is to solve the problems of the prior art, according to one aspect of the present invention by optimizing the steel component to improve the resistance to front corrosion and by controlling the size and shape of the inclusions in the steel appropriately for local corrosion Improved resistance is provided for ship steels excellent in resistance to front and local corrosion even without coating or surface treatment in the brine of pH 3 or less.

The steel for ships of the present invention for solving the problems of the present invention is C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.03% by weight or less, S: 0.03% by weight % Or less, Cu: 0.05-1.0 wt%, Al: 0.1 wt% or less, N: 0.001-0.01 wt%, Ni: 0.05-3.0 wt%, Cr: 0.02-1.0 wt%, Mo: 0.02-0.5 wt%, W: 0.02 to 0.5% by weight and Ca: 0.0005 to 0.01% by weight of one or more elements selected from the group consisting of residual Fe and other unavoidable impurities, and among the above components, C, Si, Mn, Cu, Ni, Cr, Mo and W are characterized by having a composition that satisfies the relationship of 1.5> C + 2.5Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr.

At this time, it is preferable that the maximum length of the inclusion direction existing in the said steel material is 70 micrometers or less, and the ratio (shape ratio) of the maximum width in a rolling direction and the direction perpendicular | vertical to a rolling direction is 30 or less.

In addition, the components of the steel is effective to satisfy the relationship between Ca / O> 0.25, where Ca and O respectively represent the content (% by weight) of the corresponding element contained in the steel.

And it is preferable that the area fraction of CaS inclusions in all the inclusions is 20% or more.

According to the present invention, by optimizing the steel composition and appropriately controlling the size and shape of the inclusions in the steel, the corrosion rate of the front surface is significantly lower in the strong acid saline solution having a pH of 3 or less, and no cavities are generated in the solution, thereby preventing local corrosion. It is excellent, and there is an effect of providing a ship steel material exhibiting excellent front and local corrosion resistance, especially in the environment, such as tanker tanks.

Hereinafter, the present invention will be described in detail.

In general, low alloyed steels are susceptible to corrosion in the brine atmosphere. The corrosion rate depends on the corrosion environment, ie the concentration of salt, temperature and pH of the solution. The higher the salt concentration, the higher the temperature of the solution, the more acidic the solution, i.e. the lower the pH, the faster the corrosion rate.

In particular, since most of the steel used in ships are corroded by seawater, there is no significant difference in salinity or salt acidity except for a slight temperature difference depending on the environment of the ship site. However, the oil for oil tankers of oil tankers is not corroded by seawater, but by saltwater contained in crude oil, and the brine has a high salt concentration compared to seawater. The analysis revealed. When the crude oil is loaded into the crude oil tank, the bottom surface of the crude oil tank is formed with a thin crude oil coating layer, and when the crude oil loading is completed, the brine included in the crude oil due to the specific gravity difference is formed in the lower layer. Crude coating layer produced during crude oil loading serves to prevent corrosion, so the progress of corrosion is delayed. However, corrosion is first started at the part where the crude oil coating layer is damaged by sludge included in the crude oil flow rate. At the site of corrosion, the hydrogen generated by the corrosion reaction and chlorine ion in the brine are electrically coupled to increase the acidity in the solution, and the corrosion progresses rapidly as the acidity in the solution increases. In comparison, corrosion proceeds at a much faster rate. Sudden corrosion in the areas where the coating layer is damaged appears in the form of local corrosion, but strictly different from local corrosion mechanism caused by galvanic corrosion.

Therefore, the type of corrosion that occurs in bottom oil of crude oil tank is not a local corrosion due to the characteristics of steel, but a simple form of corrosion caused by external factors. Therefore, if the oil tank is in contact with a low pH saline solution, it is resistant to corrosion. It was a general opinion that if it was raised, it could be solved.

However, the study and experiment for the present invention it was found that even in a low pH saline solution according to the characteristics of the steel local corrosion as a formula, the degree is very serious. Particularly, it was a general recognition that local corrosion such as formula is only a problem in austenitic stainless steels, but is not a big problem in general low alloy steels. According to the research results of the present inventors, local corrosion is not affected even in low alloy steels. Seriously could happen. The characteristics of the steel affecting local corrosion include surface cracks and precipitates that occur depending on the components, and the local corrosion characteristics may be completely changed depending on the size and shape of the inclusions.

Therefore, the present inventors have repeatedly conducted research and experiments to overcome the problems not found in the prior art, and as a result, C, Mn, Cu, Ni, Cr, By optimizing the components such as Mo, W, and limiting the formation of stretched oxide-based inclusions widely distributed in the form of steel sheets in the steel species, corrosion resistant steels for ships having excellent corrosion resistance and local corrosion in a low pH salt solution were prepared. .

Hereinafter, the component system of the ship steel of this invention is demonstrated first.

C: 0.02 to 0.2 wt%

The C is an element added to increase the strength to increase the hardenability by increasing the content, but the strength is increased as the addition amount increases, inhibiting the corrosion resistance of the front surface, and promotes precipitation of carbides, etc. It also has some effect on resistance. To improve front and local corrosion resistance, C content should be reduced. However, if C is less than 0.02% by weight, it is difficult to secure the strength. If it exceeds 0.2% by weight, the weldability is deteriorated, which is undesirable as a steel for welding structures. The range is limited to%. It is more preferable to make C 0.12 weight% or less from a corrosion resistance viewpoint.

Si: 0.05-1.5 wt%

The Si is required to be 0.05% by weight or more in order not only to act as a deoxidizer but also to increase the strength of the steel. In addition, it is advantageous to increase the content because Si contributes to the improvement of the front corrosion resistance, but if the content of Si exceeds 1.5% by weight, the toughness and weldability are inhibited and the peeling of the scale during rolling is difficult. Since it causes surface defects, it is preferable to limit the content to 0.05 to 1.5% by weight. In order to improve corrosion resistance, it is more preferable to add 0.2 wt% or more of Si.

Mn: 0.2-2.0 wt%

Mn is required at least 0.2% by weight to secure strength. If the content is increased, the hardenability is increased to increase the strength, but when added in excess of 2.0% by weight, there is a problem that the weldability is lowered, it is preferable to limit the content to 0.05 to 2.0% by weight. Mn does not have a significant effect but affects the corrosion resistance of the front face. It is more preferable to add Mn at 1.5 weight% or less from the viewpoint of the front corrosion resistance.

P: 0.03 wt% or less

P is an impurity element, and if the content is added in excess of 0.03% by weight, not only the weldability is significantly lowered but also the toughness is degraded, so the content is preferably limited to 0.03% by weight or less.

S: 0.03 wt% or less

S is also an impurity element and if its content exceeds 0.03% by weight, there is a problem of deteriorating ductility, impact toughness and weldability of steel. Therefore, it is desirable to limit the content to 0.03% by weight or less. In particular, S is easily reacted with Mn to form stretched inclusions, such as MnS, and since the vacancy present at both ends of the stretched inclusions may be a local corrosion start point, the content is more preferably limited to 0.005% by weight or less.

Cu: 0.05-1.0 wt%

When Cu is contained in an amount of 0.05% by weight or more together with Ni and Cr, it is effective for improving the front corrosion and local corrosion resistance by delaying the dissolution of Fe. However, if it exceeds 1.0% by weight, it may cause surface cracking during slab manufacture, thereby lowering the resistance to local corrosion, and when reheating the slab for rolling, Cu having a low melting point may penetrate into grain boundaries of the steel, causing cracks during hot working. It is preferable to limit the content to 0.05 to 1.0% by weight. Since surface cracks generated during slab production interact with each other with C, Ni, and Mn contents, the occurrence frequency of surface cracks may vary depending on the content of each element, but the Cu content is most preferably 0.5% by weight or less.

Al: 0.001-0.1 wt% or less

Al is an element that is necessarily added for deoxidation to react with N in the steel to form AlN to refine the austenite grains to improve toughness. At least 0.001% by weight must be added for deoxidation. However, when excessively contained in excess of 0.1% by weight, the inclusions are formed in the coarse oxide in the steelmaking process. The formation of the stretch inclusions encourages the formation of cavities around the inclusions, which act as a starting point for local corrosion and thus serve to inhibit local corrosion resistance. Therefore, it is preferable to limit Al content to 0.1 weight% or less.

N: 0.001-0.01 wt%

Since N is difficult to remove industrially completely from steel, N is made into the lower limit 0.001 weight% which is the range which can accept the load in a manufacturing process. N forms nitrides with Al, Ti, Nb, V, etc., which hinders austenite grain growth, which helps to improve toughness and strength. Since N in these solid solution states adversely affect the toughness, it is preferable to limit the range to 0.001 to 0.01 wt%.

In addition to the above-mentioned advantageous composition, at least one of Ni, Cr, Mo, W and Ca to be described below needs to be included.

Ni: 0.05-3.0 wt%

When Ni is contained in an amount of 0.05% by weight or more like Cu, it is effective for improving the front corrosion and local corrosion resistance. In addition, when added together with Cu, there is an effect of suppressing the problem of cracking during hot working by suppressing the formation of a low melting point Cu phase by reacting with Cu. Ni is also an effective element for improving the toughness of the base material. However, since it is an expensive element, the addition of more than 3.0% by weight is disadvantageous in terms of economics or weldability, so it is preferable to limit the content to 0.05 to 3.0% by weight. Since the effect of Ni on the corrosion resistance is not higher than that of Cu, it is more preferable to contain 1 to 5 times the Cu content to suppress the surface cracking due to the addition of Cu rather than adding a large amount to improve the corrosion resistance.

 Cr: 0.02-1.0 wt%

The effect of Cr is not great but not only improves the corrosion resistance of the front surface but also contributes to the improvement of strength. In order to show the effect of the addition of Cr, it should be contained 0.02% by weight or more. However, if excessively contained in excess of 1.0% by weight, rather than encourage local corrosion, such as pitting (pitting) and adversely affect the toughness and weldability, it is preferable to limit the content to 0.02 ~ 1.0% by weight.

Mo: 0.02-0.5 wt%

Mo is an element contributing to the improvement of corrosion resistance and strength and should be added at least 0.02% by weight in order to exhibit the effect. However, Mo must be employed in steel to improve corrosion resistance. Mo contained more than the solid solution can contribute to the increase of strength by forming precipitates, but these precipitates can form a ferrite and galvanic rather increase the corrosion rate, and if the precipitate is coarse, the pores formed in the precipitate and steel interface It acts as a starting point for local corrosion and reduces local corrosion resistance. Therefore, the upper limit is preferably 0.5% by weight or less. Therefore, Mo content is limited to 0.02 to 0.5% by weight. The high capacity of Mo can be increased by controlling the steelmaking or rolling process. However, since Mo can be dissolved in 0.1% by weight without particular limitation on the process, Mo is more preferably added in 0.1% by weight or less.

W: 0.02-0.5 wt%

W is an element that plays the same role as Mo and contributes to the improvement of corrosion resistance and strength and should be added in an amount of 0.02% by weight or more. However, like Mo, W must be dissolved in steel to improve corrosion resistance. W contained above the solid solution limit may form precipitates and contribute to the improvement of strength, but these precipitates may form ferrite and galvanic to increase the corrosion rate, and if the precipitates are coarse, the pores formed in the precipitate and steel interface It acts as a starting point for local corrosion and reduces local corrosion resistance. Therefore, the upper limit is preferably 0.5% by weight or less. Therefore, the W content is limited to 0.02 to 0.5% by weight. The high capacity of W can be increased by controlling the steelmaking or rolling process, but the high capacity of W is smaller than that of Mo, and thus the amount of W that can be dissolved without particular restriction on the process is 0.05% by weight, more preferably W Is controlled to 0.05% by weight or less.

Ca: 0.0005 ~ 0.01 wt%

The Ca is effective for controlling the inclusions, thus suppressing the formation of the stretch inclusions, and CaO, CaS, produced by the addition of these elements is easily dissolved in the solution, which delays the acidification of the solution. In order to exhibit these characteristics, it should contain 0.0005% by weight or more. On the other hand, the upper limit is limited to 0.01% by weight, in which the inclusions are excessively coarse to be in a range that spoils local corrosion resistance, ductility and toughness. Therefore, the Ca is preferably limited to 0.0005 to 0.01 wt%.

On the other hand, as described above, Cu, Ni, Cr, Mo, W and the like are elements that contribute to the improvement of the front corrosion and local corrosion resistance, but may also promote local corrosion depending on the content of each element. This is because surface cracks and coarse precipitates, which may be the starting point of local corrosion, depend on the content of each alloying element.

Surface cracking is a phenomenon that occurs when the aposperm section is excessive during continuous casting. The range of the aposphere section changes depending on the content of the added alloying element. C, Mn, Cu, Ni, etc. are elements that promote surface crack formation by extending the reaction zone with austenite stabilizing elements, while Si, Mo, W, and Cr reduce the reaction zone with ferrite stabilization elements, forming surface cracks. Becomes an element that suppresses However, among these, the influence on the surface cracking is slightly different. Among these elements, C also affects the range of the trapping reaction zone, but it is important to determine whether or not the passage of the apolytic reaction zone is ultimately determined by the amount of C. It is known that surface cracks are significantly affected by Cu and Ni contents, which are austenite stabilizing elements, rather than stabilizing elements.

Unlike the effect of each alloying element on the surface cracking, precipitate formation is related to the solid solution limit of ferrite stabilizing elements. Mo, W, Cr, etc. are elements that can form carbides by reacting with C. Especially, W, Mo has a very high solid solution limit in steel, and if it is excessively contained over a certain range, it forms coarse carbides. Can act as a point of view. On the other hand, Cr has a higher solubility limit than Mo and W, so it is difficult to form carbide unless it contains a large amount.

As described above, it is difficult to theoretically quantify the effects of not only different surface cracks or precipitate formation on each alloy element, but also there is interaction between alloy elements on these properties. Accordingly, the present inventors have derived an empirical formula as shown in Equation 1 in consideration of the influence of each element on the surface crack and precipitate formation.

1.5> C + 2.5 Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr

However, here, C, Si, Mn, Ni, W, Cu, Mo, Cr means the content (wt%) of each component.

Equation 1 is a condition in which local corrosion does not occur in consideration of the influence of each component, and when the value of the right side (also called fitting index) of Equation 1 is 1.5 or more, the content of each element is described above. Even if it meets the range, local corrosion occurs rapidly and the area occupied by local erosion after 144 hours immersion in 10% NaCl solution with pH 1 exceeds 10%. However, when the component range of the present invention is satisfied and the condition of Equation 1 is satisfied, the progress of local corrosion can be significantly prevented.

In addition, according to the results of the present inventors, it is more preferable to control them because inclusions of various sizes generated during deoxidation in addition to surface cracks and coarse precipitates caused by alloying elements affect local corrosion.

Oxide inclusions produced during the steelmaking process have various compositions and sizes. These various inclusions are present in the steel in various forms due to the deformations applied during rolling. Small spherical inclusions retain their shape during rolling and are in close contact with the steel so that no voids form around the inclusions. On the other hand, coarse oxide inclusions that are easily broken or coarse inclusion clusters of several inclusions are easily broken during rolling and stretched inclusions that are elongated along the metal movement during rolling. The stretched inclusions that are long and broken due to the large inclusions have an irregular shape and thus are not in close contact with the steels, and a void is created between the inclusions and the steels. When immersed in a corrosion solution, the corrosion progresses rapidly around these public areas, resulting in local corrosion.

Therefore, the size and shape of the inclusions must be controlled in order to produce steel having excellent local corrosion resistance. The present inventors have repeatedly observed the inclusions and stretched inclusions having a length of 70 μm or more along the rolling direction, or the ratio (shape ratio) of the maximum width in the rolling direction and the direction perpendicular to the rolling direction is 30. It was confirmed that local corrosion was initiated around these inclusions if stretched inclusions were present. Therefore, in order to secure excellent local corrosion resistance, the maximum inclusion length in the rolling direction should be limited to 70 μm or less, and the ratio of the maximum length in the rolling direction and the maximum width in the direction perpendicular to the rolling direction does not exceed 30. It is desirable to limit the number of times.

The method commonly used to control the size and shape of the oxide inclusions in the final product is to separate the coarse inclusions as much as possible in the steelmaking process, leaving only the fine spherical inclusions in the molten steel and casting the slab. As mentioned above, the fine spherical inclusions are broken during rolling and are not long and do not form a long line, so they maintain good adhesion with the steel, so that no void is formed between the steel and the inclusions, and thus does not create a place where local corrosion can be started. In order to fully remove and remove coarse inclusions in the steelmaking process, bubbling must be carried out at least 5 minutes before continuous casting after the converter operation.

Another method to control the size and shape of the oxide inclusions in the final product is a method of adding Ca to react with Al-based oxides to produce inclusions with low melting point to make the liquid phase in the molten steel. In general, liquid inclusions are more easily separated than solid phase inclusions, and thus, Ca can be added to remove coarse Al oxide inclusions, and as is well known, Ca plays a role of spheroidizing inclusions, thereby removing local corrosion initiation points. effective. In addition, CaO or CaS inclusions are easily dissolved in the solution, thereby increasing the pH of the solution. Therefore, when a large amount of CaO or CaS inclusions plays a role in increasing the pH of the solution, it is also possible to increase the front corrosion resistance of the steel. The effect of this Ca input is a significant difference depending on the Ca input method and the present invention will be described with respect to a more preferable Ca input method that can maximize the Ca input effect.

In general, Ca is a highly reactive element, which easily reacts with Al oxide remaining in molten steel when it is injected into molten steel to form various types of complex inclusions, but since the 12Al ₂ O ₃ -7CaO compound has the lowest melting point, liquid inclusion formation is difficult. Easy and easy to separate

However, when the other oxides are removed from the molten steel, the dissolved oxidation amount in the molten steel is also reduced, thereby facilitating the generation of CaS.

Therefore, the present inventors conducted research and experiment to improve the steelmaking process in the direction of promoting the formation of CaS inclusions in the molten steel, and as a result, instead of the conventional method of inserting the conventional Ca-Si wire, the Ca-Si wire was replaced with 2 It has been found that CaS inclusions in the steel are greatly increased by splitting more than once. In more detail, when the Ca-Si wire is divided into two or more times as described above, the initially added Ca reacts with the remaining oxide in the molten steel to form a composite inclusion, and then waits for the subsequent Ca input. Floating separation in the molten steel and mixed into the slag layer, the composite inclusions in the molten steel is reduced. Therefore, when Ca is added after two batches, Ca can react with S in molten steel to form CaS effectively.

In addition, as described above, in order to form liquid inclusions that are easily separated by flotation through Ca, and to form spherical CaS inclusions that are easily dissolved in a solution, the amount of Ca must be appropriately controlled according to dissolved oxygen. As shown in Equation 2 below, when the ratio of Ca in the molten steel and the dissolved oxygen ratio during Ca input exceeds 0.25, the liquid inclusions start to form, and as the ratio increases, the fraction of the liquid inclusions gradually increases, so that formation of coarse inclusions is suppressed. Therefore, in order to obtain a local corrosion resistance effect according to Ca addition, the ratio of Ca in molten steel and dissolved oxygen at the time of Ca injection should be limited to 0.25 or more, more preferably 0.5 or more.

Ca / O> 0.25

Here, Ca and O respectively mean the content (% by weight) of the corresponding element contained in the steel.

In the present invention, the steel slab is manufactured by continuous casting of molten steel whose composition is controlled as described above, and then hot rolled under normal conditions so that the maximum inclusion length in the final steel product does not exceed 70 μm or the maximum inclusion length and inclusions. Steels for ships with excellent front and local corrosion resistance can be produced in strong acid saline solutions with a pH of 3 or less, which does not exceed 30.

In order to improve not only local corrosion resistance due to Ca addition but also front corrosion resistance, it is more preferable that the ratio of Ca and dissolved oxygen (ie, Ca / O) is greater than 0.75 when Ca is injected. If the above criteria are met, spherical fine CaS is produced together with the liquid inclusions.

In the present invention, in order to improve the front corrosion resistance according to the Ca input, the area of the CaS-based non-metallic inclusions is controlled to be 20% or more of the total inclusions. If the fraction is less than 20%, the pH of the steel surface can be effectively obtained. Because you can't.

That is, Ca introduced into the molten steel reacts with oxygen, aluminum, silicon, sulfur, etc. present in the molten steel to form inclusions (Al, Si, Ca) O and CaS. Of these inclusions, except for CaS, the complex inclusions do not dissolve in the aqueous solution and thus do not contribute to the pH increase of the steel surface, and only CaS inclusions are dissolved in the aqueous solution to raise the pH. Therefore, the ratio of CaS in the total inclusions is a major factor in determining the degree of rise of the pH of the solution, and the pH of the water film can be contributed to improving the corrosion resistance only when the corrosion rate is 3.0 or more. In order to obtain a pH of 3.0 or more as described above, the area fraction of CaS inclusions in the total inclusions should be 20% or more.

More preferably, the area fraction of the water-soluble CaS inclusions in the Ca-based nonmetallic inclusions in steel is limited to 20 to 80%. The reason for this is as follows. When Ca is added, oxide inclusions and emulsion inclusions are simultaneously produced. CaS, an emulsion-based inclusion, may cause nozzle clogging in the continuous casting process, and oxide-based inclusions may cause melting of the refractory in the continuous casting process. Therefore, in order to improve both weather resistance and secure the stability of the continuous casting process, it is more preferable to limit the upper limit of the fraction of CaS inclusions in the total inclusions to 80% or less.

As described above, the present invention optimizes the steel components, and by appropriately controlling the fraction of CaS inclusions in the Ca-based non-metallic inclusions in the product to provide a ship steel material excellent in corrosion resistance to the front or local corrosion in a strong acid salt solution of pH 3 or less It can manufacture.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

Example 1

To prepare a molten steel having a composition as shown in Table 1 below to prepare a steel slab by using a continuous casting. In order to control the maximum size of inclusions affecting local corrosion in steel slab manufacturing to 70㎛ or less, and to control the aspect ratio to 30 or less, bubbling was carried out for 5 minutes or more to separate the floating parts before casting. The steel was manufactured by hot rolling under the conditions. Meanwhile, Comparative Steel S of Table 1 is a general ship structural steel and was prepared for comparison with the present invention steel in this embodiment.

In Table 1, the fitting index means C + 2.5Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr, which is the right side of Equation 1.

As can be seen in Table 1, the invention steel A to invention steel R is not only satisfy the component range defined in the present invention, but also the fitting index is less than 1.5 is a steel material that satisfies all the composition control conditions of the present invention. In addition, by performing bubbling for 5 minutes or more, the maximum inclusion size is all 70 µm or less, and the aspect ratio is also 30 or less, which is a steel material that satisfies all the preferable conditions of the present invention. However, Comparative steel S is a component of the conventional marine steels, not only does the carbon content not meet the conditions defined in the present invention, Cu is not added at all, and 1 of Cr, Ni, Mo, W, and Ca. It shows the case where no element or more of a kind is added at all. Comparative steel T to comparative steel W is a case in which Cu is not added and deviates from the component specification defined in the present invention. Comparative steel X is not only for the case where Cu is not added, but also for the case where the Mo content is excessive. The case is shown. In addition, the comparative steel Y, comparative steel AA to comparative steel FF is the component range meets the range required by the present invention, but the fitting index is all 1.5 or more, even if the component range is satisfied even if the fitting index is not satisfied for local corrosion This is a case to confirm that resistance is low.

As shown in Table 2 below, by adding HCl to a 10% NaCl solution by weight percent corrosion rate shown in Table 2, 144 hours of invention steel and comparative steel were immersed in a 30 ° C. solution adjusted to pH 0.85, and then the weight loss was measured. After conversion, the corrosion rate of the front surface was calculated and the relative value of the corrosion rate of the comparative steel S, which is a general steel for ships, was expressed as 100. The food pore fraction shown in Table 2 below is a value obtained by dividing the area of the food pore by assuming that the food pore is a circle among the total specimen surface areas.

division Relative Corrosion Rate Food fraction Inventive Steel A 35.2 0 Inventive Steel B 22.7 1.1 Invention Steel C 23.7 0.1 Inventive Steel D 15.1 0 Inventive Steel E 15.4 1.1 Inventive Steel F 30.4 0.7 Invention Steel G 12.8 0 Inventive Steel H 20.4 2.2 Inventive Steel I 22.3 0.2 Invention Steel J 13.1 0.5 Inventive Steel K 35.4 1.2 Inventive Steel L 29.8 0.5 Inventive Steel M 35.9 2.5 Inventive Steel N 30.1 0.4 Invention Steel O 22.2 2.1 Inventive Steel P 25.4 1.9 Invention Steel Q 24.7 0 Inventive Steel R 12.5 0.8 Comparative Steel S 100 0.2 Comparative Steel T 95.4 24.7 Comparative Steel U 92.1 15.3 Comparative Steel V 99.7 0.8 Comparative Steel W 98.2 27.8 Comparative Steel X 101.5 0 Comparative Steel Y 85.1 32.8 Comparative Steel Z 87.6 30.1 Comparative Steel AA 88.7 31.4 Comparative Steel BB 91.2 27.9 Comparative Steel CC 89.1 22.4 Comparative Steel DD 86.4 15.6 Comparative Steel EE 90.2 12.1 Comparative Steel FF 64.1 18.7

As can be seen in Table 2, in the case of the comparative steel S, in which Cr, Ni, Cu, Mo, W, and the like are not added, the food content ratio is not high, but the relative corrosion rate is significantly higher than that of the invention steel. In other words, it is necessary to suppress the front corrosion by the addition of such elements, but in the comparative steel S without addition of the element, the front corrosion occurs easily. A large corrosion rate was observed in the comparative steels T to X without Cu addition. However, the comparison steel T, comparative steel U, comparative steel W of these steels can be confirmed that the food content is also increased at the same time as the fitting index is greater than 1.5. In addition, the comparative steel Z is not only excessive W content, but also a case where the fitting index is 1.5 or more, and the food content fraction and the front corrosion rate are high.

The relationship between the fitting index of Table 1 and the food porosity of Table 2 is shown in FIG. As shown in the figure, the food content fraction is less than 3% until the fitting index reaches 1.5, but when the fitting index reaches 1.5, the food fraction increases abruptly, and the tendency increases as the fitting index increases. The fraction also tends to increase linearly. That is, when the fitting index exceeds 1.5, it can be seen that the local corrosion resistance suddenly decreases. Therefore, in order to obtain good local corrosion resistance, the fitting index should be limited to less than 1.5. In addition, Fig. 2 shows the relationship between the fitting index and the front corrosion speed, as shown in the figure, when the fitting index is 1.5 or less, as shown in the drawing, it appears as 30% or less of the general steel, but when the fitting index is 1.5 or more, The speed increases rapidly, reaching a level similar to that of ordinary steel.

Example 2

After melting the steel having the same composition as the invention steel prepared in Example 1, and then casting a different bubbling time for the separation of the inclusions before casting to manufacture a slab and then hot-rolled the slab under normal conditions Steel sheet was prepared. At this time, the maximum inclusion length and shape ratio (inclusion length / width) were measured by analyzing the size distribution of the inclusions in the steel, and the food fraction was measured after the immersion test.

Steel grade division Bubbling time Maximum length of inclusions (mm) Maximum aspect ratio (length / width) Food fraction (%) Inventive Steel A Inventory 1 5 minutes 12 2.1 0.0 Inventive Steel A Comparative Example 1 3 minutes 57 34 4.2 Inventive Steel B Inventive Example 2 5 minutes 7 1.5 1.1 Inventive Steel B Comparative Example 2 2 minutes 68 54 4.5 Inventive Steel E Inventory 3 6 minutes 9 1.9 1.1 Inventive Steel E Comparative Example 3 2 minutes 46 31 6.1 Invention Steel G Honorable 4 5 minutes 14 3.1 0.0 Invention Steel G Comparative Example 4 1 minute 82 67 5.6 Inventive Steel H Inventory 5 7 minutes 8 1.4 2.2 Inventive Steel H Comparative Example 5 1 minute 79 54 6.4 Inventive Steel K Inventory 6 6 minutes 11 3.1 1.2 Inventive Steel K Comparative Example 6 1 minute 85 64 6.1 Inventive Steel L Honorable 7 5 minutes 12 4.8 0.5 Inventive Steel L Comparative Example 7 3 minutes 63 55 3.9

As shown in Table 3, the maximum inclusion length and shape ratio are increased by optimizing the composition and bubbling time for inclusion separation during steelmaking process satisfying Equation 1, that is, in the form of stretch inclusions. It can be seen that the food content is changed and the food fraction increases. Figure 3 shows the aspect of the planting occurred in each steel after the immersion test, it can be seen that even the same invention steel according to the invention and comparative examples completely different form of the planting.

In the comparative example of the inventive steel, the reason why the area of cultivation is increased is due to the stretching inclusions as shown in Table 3 above. 4 shows a process in which local corrosion generated in a stretch inclusion of steel of Comparative Example 1 grows into a large plant. In the drawings, the photograph of the steel hole after the immersion test is a photograph observing the presence of the inclusion 3 times before the immersion test.

Comparative Example 1 Before the immersion test, when the inclusions in the steel were observed, the inclusions observed at position 3 had the shape of an elongated inclusion having a length of 110 μm while the inclusions observed at other locations had a relatively fine spherical inclusion shape. As a result of observing the shape after the immersion test of the same steel, it can be seen that the hole was generated at the position 3 with the extension inclusions. Therefore, it can be seen that the presence of the stretch inclusion promotes the formation of food balls, thereby deteriorating local corrosion resistance. Considering the maximum size and shape ratio of inclusions shown in Table 3 above, by limiting the maximum inclusion length to 70㎛ or less, or by limiting the shape ratio to 30 or less, excellent local corrosion resistance can be obtained.

Example 3

In Example 2, it was confirmed that the stretch inclusions generated inside the steel were a factor that inhibited the local corrosion resistance. As a method for realizing this, when bubbling before casting in the molten steel was sufficiently performed, the large inclusions were separated and removed to remove the internal inclusions. No stretch inclusions are produced at. As a method of flotation and separation of large inclusions, there is a method to facilitate flotation by generating inclusions having low melting point in molten steel by adding Ca. Low melting point inclusions exist in the liquid phase in the molten steel and facilitate flotation. The formation of such liquid inclusions is related to the amount of Ca and the amount of dissolved oxygen in molten steel when Ca is added. 5 is a graph predicting the type of inclusions formed according to Ca input amount and dissolved oxygen content in molten steel through thermodynamic calculation.

The molten aluminate (molten aluminate) shown in Figure 5 is a liquid inclusion with a low melting point, the liquid inclusions start to generate when [Ca] / [O] exceeds 0.25, the amount of liquid inclusions increases as the value increases Therefore, [Ca] / [O] should exceed 0.25 to suppress the formation of stretch inclusions. When the value of [Ca] / [O] exceeds 1, spherical CaS inclusions are formed. CaS is easily dissolved in the solution and increases the pH of the solution. Table 4 shows the Ca input amount, dissolved oxygen amount, inclusion maximum length and shape ratio, CaS inclusion fraction, front corrosion rate, and food pore fraction of Ca-added inventive steel and comparative steel.

Steel grade Ca amount
(weight%) Dissolved oxygen
(weight%) [Ca] / [O] Maximum length of inclusions (mm) Maximum aspect ratio of inclusions
(Length / width) CaS fraction
(%) Food fraction
(%) Invention Steel O 0.001 0.0026 0.43 12 1.3 25 0.0 Inventive Steel P 0.0015 0.0027 0.55 6 One 38 0.0 Invention Steel Q 0.0007 0.0022 0.31 31 14 21 0.8 Inventive Steel R 0.0013 0.0032 0.41 13 1.4 26 0.2 Comparative Steel EE 0.0004 0.0030 0.13 73 37 9 8.7 Comparative Steel FF 0.0005 0.0032 0.16 64 35 11 9.4

As shown in Table 4, small inclusions close to the spherical shape were observed in the inventive steel exceeding [Ca] / [O] by 0.25 or more, while stretch inclusions were observed in the comparative steel having [Ca] / [O] of less than 0.25. Due to the effect of inhibiting the formation of stretched inclusions due to the addition of Ca, the food content ratio between the inventive steel and the comparative steel shows a big difference. That is, since Ca is added so that [Ca] / [O] exceeds 0.25, excellent local corrosion resistance can be obtained. In addition, when the [Ca] / [O] ratio is high, it was also confirmed that the CaS ratio also increases to contribute to increase the pH of the solution.

1 is a graph showing the correlation between the fitting index and the degree of food hole generation,

2 is a graph showing the correlation between the fitting index and the relative corrosion rate;

Figure 3 is a photograph showing the difference in the tendency to appear when the steel is manufactured when the steel is manufactured by varying the bubbling time for the same molten steel,

Figure 4 is a micrograph observing the phenomenon that the food is generated in the elongated inclusions during the immersion test, and

5 is a graph showing the relationship between the appropriate Ca and O content for forming molten aluminate in molten steel.

Claims (4)

C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.03% by weight or less, S: 0.03% by weight or less, Cu: 0.05-1.0% by weight, Al: 0.1% by weight % Or less, N: 0.001-0.01 weight%, Cr: 0.1-1.0 weight%, Ni: 0.05-3.0 weight%, Mo: 0.02-0.5 weight%, W: 0.02-0.5 weight% and Ca: 0.0005-0.01 weight Consisting of at least one element selected from the group consisting of%, balance Fe and other unavoidable impurities, wherein C, Si, Mn, Cu, Ni, Cr, Mo and W are 1.5> C + 2.5Si + 0.3Mn + A ship steel material having a composition that satisfies the relationship between 4Ni + 3W-3Cu-4Mo-6Cr and having excellent front and local corrosion resistance in a strong acid saline solution, characterized in that an area fraction of CaS inclusions in the total inclusions is 20% or more. 2. The maximum length in the rolling direction of the inclusions present in the steel is 70 µm or less, and the ratio (shape ratio) of the maximum width in the rolling direction and the maximum width in the direction perpendicular to the rolling direction is 30 or less. Steel for ships with excellent resistance to front and local corrosion in strong acid brine solution. delete 2. The front surface of a strong acid saline solution according to claim 1, wherein a content of Ca / O> 0.25 is satisfied, wherein Ca and O each represent a content (% by weight) of a corresponding element contained in the steel. Marine steel with excellent corrosion and local corrosion resistance.

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